Wind turbine generators that generate electricity by means of natural energy, namely, wind force, are conventionally known. In one wind turbine generator, for example, a rotor head equipped with wind turbine rotating blades, a main shaft coupled to the rotor head so as to rotate together therewith, a gearbox to which the main shaft is coupled so as to rotate as the wind turbine rotating blades receive wind force, and a generator driven by the shaft output of the gearbox are provided on a nacelle mounted on a tower. In the thus-configured wind turbine generator, the rotor head equipped with the wind turbine rotating blades, which convert wind force into torque, and the main shaft are rotated to generate a shaft output, and the gearbox coupled to the main shaft transfers the shaft output to the generator while increasing the rotational speed thereof. Thus, the shaft output generated by converting wind force into torque serves as a driving source for the generator, and wind force can be used as a power source for the generator to generate electricity.